WO2018068991A1 - Method for producing a stress-decoupled micromechanical pressure sensor - Google Patents

Abstract

The invention relates to a method for producing a micromechanical pressure sensor (100), comprising the steps: providing an MEMS wafer (10) having a silicon substrate (11) and a first cavity (13) formed therein under a sensor membrane (12); providing a second wafer (30); bonding the MEMS wafer (10) to the second wafer (13), and exposing a sensor core (12, 13, 13a) from the rear side, wherein a second cavity (18) is formed between the sensor core (12, 13, 13a) and the surface of the silicon substrate (11), wherein the second cavity (18) is formed by means of an etching process, which is carried out with etching parameters changed in a defined manner.

Description

 description

Title METHOD FOR MANUFACTURING A STRESSENTKOPPELTEN

 MICROMECHANICAL PRESSURE SENSOR The invention relates to a method for producing a micromechanical

Pressure sensor. The invention further relates to a micromechanical pressure sensor.

State of the art

Micromechanical pressure sensors in which a pressure difference is measured as a function of a deformation of a sensor membrane are known, for example, from DE 10 2004 006 197 A1. Semiconductor resistors, which are used in known micromechanical pressure sensors as mechanical-electrical converter, not only absorb the mechanical stress that results from the influence of pressure on the membrane, but also stress due to mechanical disturbances. The two most important disturbances are the following:

- Incorporated stress on the part of the assembly and connection technology (AVT) through:

 - Deformation of the PCB on which the sensor is mounted

 Deformation of the housing over temperature, e.g. by soldering

 - Temperature behavior of a used adhesive

- Intrinsic stress from the sensor element due to temperature behavior of cover layers Some of these effects can be controlled by a suitable balance, for example in the case of the dielectric cover layers. The influence The metallization and the AVT-side coupled stress is subject to a significant dependence on a history of the component (eg due to creep of the metal, in particular during / after soldering). This can not be compensated by a comparison before delivery of the components.

DE 10 2015 1 16 353 A1 discloses a microintegrated encapsulated MEMS sensor with mechanical decoupling and a manufacturing method therefor. At this time, the pressure sensor made on an SOI substrate is removed from the back by etching out the buried oxide layer, e.g. freed by HF gas-phase etching. The supply of the etching gas is carried out by previously penetrated into the silicon of the wafer back access holes.

Disclosure of the invention

It is an object of the present invention, an alternative manufacturing method for a micromechanical pressure sensor with improved

To provide operating behavior.

The object is achieved according to a first aspect with a method for producing a micromechanical pressure sensor, comprising the steps:

 Providing a MEMS wafer having a silicon substrate and a first cavity formed therein under a sensor membrane;

 Providing a second wafer;

 Bonding the MEMS wafer to the second wafer;

 Forming a sensor membrane in the silicon substrate; and

 Cropping a sensor core from the back, wherein a second cavity between the sensor core and the surface of the silicon substrate is formed, wherein the second cavity is formed by means of an etching process, which is carried out with defined changed Ätzparametern.

In this way, a micromechanical pressure sensor is provided with an all-sided pressure sensor membrane, whereby an efficient stress decoupling structure is realized. By changing an etching regime, it is possible to provide the second cavity required for this purpose within the silicon substrate. In this way, at the same time, become more effective Protection against contamination and media access created for the micromechanical pressure sensor. False signals of the micromechanical pressure sensor can be greatly reduced in this way, whereby an operating characteristic of the pressure sensor is improved. By means of the supporting action of the second wafer, which is bonded to the MEMS wafer, the entire

Process can be done from the back in a comfortable way.

According to a second aspect, the object is achieved with a micromechanical pressure sensor, comprising:

 a pressure sensor core formed in a silicon substrate with a

Sensor membrane, wherein in the sensor membrane, a first cavity is formed;

 a second cavity formed in the silicon substrate above the sensor core;

 - wherein the second cavity was produced by means of an etching process,

 whose etching parameters were changed in a defined manner during the etching process.

Preferred embodiments of the method for producing a micro-mechanical pressure sensor are the subject of dependent claims.

An advantageous development of the method provides that for forming the second cavity, the etching process is a reactive ion deep etching process with anisotropic onset and defined isotropic continuation. In this way, known etching processes are advantageously used to form the second

Cavity used.

A further advantageous development of the method provides that for forming the second cavity by means of a vertical etching process

Access openings are produced in the silicon substrate, wherein the vertical

Etching process is changed into a lateral etching process, wherein from the lateral etching process resulting spherical etching fronts grow together. As a result, specific properties of known etching processes for providing the second cavity are advantageously utilized.

A further advantageous development of the method is characterized in that, after the reactive ion deep etching process, a passivating and sputtering Part of the etching process is turned off. As a result, measures for adapting known etching processes to create the second cavity are made in an effective manner.

A further advantageous development of the method provides that the entire etching process is defined isotropically defined from the beginning. In this way, an alternative production method is advantageously provided.

A further advantageous development of the method provides that by means of the etching process from the beginning downwards widening, trapezoidal etching fronts are formed. In this way, advantageously provided is a downwardly formed trapezoidal widening etching regime, whereby advantageously an introduction of etching gas is facilitated. In addition, etching times can be reduced and the entire process can be better controlled.

A further advantageous development of the method provides that the second wafer is a passive substrate wafer or an ASIC wafer. As a result, different possibilities for the formation of the second wafer are advantageously realized.

A further advantageous development of the method provides that the micromechanical pressure sensor is designed as a piezoresistive pressure sensor or as a capacitive pressure sensor. In this way, different technological types of micromechanical pressure sensors can be realized.

The invention will be described below with further features and advantages with reference to several figures in detail. Same or functionally identical elements have the same reference numerals. The figures are particularly intended to illustrate the principles essential to the invention and are not necessarily to scale. For better clarity, it can be provided that not all the figures in all figures are marked.

Disclosed method features are analogous to corresponding disclosed device features and vice versa. This means in particular special, that features, technical advantages and embodiments relating to the method for producing a micromechanical pressure sensor in an analogous manner from corresponding designs, features and technical advantages of the micromechanical pressure sensor arise and vice versa.

In the figures shows:

Fig. 1 ... 4 cross-sectional views for explaining the proposed

 Method for producing a micromechanical

 Pressure sensor;

5 and 6 are cross-sectional views for explaining an alternative

 Method for producing a micromechanical

 Pressure sensor;

Fig. 7 and 8 possible finishes of the proposed micromechanical pressure sensor; and

9 shows a basic sequence of an embodiment of a

 Method for producing a micromechanical

 Pressure sensor.

Description of embodiments

A central idea of the present invention is to provide an improved manufacturing method for a micromechanical pressure sensor. For this purpose, a specifically designed etching method is proposed, which can be carried out with a simple, inexpensive Si substrate.

This achieves an efficient stress decoupling structure, which also realizes a protective device for external particles or external moisture for the micromechanical pressure sensor.

For better clarity, non-essential structures or elements are not explained in detail below for the invention. Fig. 1 shows a cross-sectional view of a first wafer 10 with a

Silicon substrate 1 1, on which a dielectric cover layer 15 is arranged. Within the silicon substrate 1 1 first access openings 14 and further a first cavity 13 are formed. Below the first cavity 13, bulk silicon 13a can be seen. A metallization element 16 serves for the following

Forming a eutectic bond 20 with a second wafer 30.

Fig. 2 shows the arrangement of Fig. 1 rotated by 180 degrees, wherein the first wafer 10 is now preferably eutectically bonded to a second wafer 30, whereby a bonding compound 20 has been created. The second

Wafer 30 may be formed as an ASIC wafer or alternatively as a passive substrate wafer (not shown). In the case of an ASIC wafer, the second wafer 30 comprises an ASIC substrate 31 and a device arranged thereon

Functional Layer 32. The stack formed in this way can be thinned back (eg by grinding) in order to shorten the processing time in the subsequent structuring step and to reduce the component height.

FIG. 3 shows a cross-sectional view through the two bonded wafers 10, 30. In a next step, a hole pattern is formed by means of photolithography

Silicon substrate 1 1 generated. The said hole pattern is then etched, preferably by means of a vertical or anisotropic etching process, preferably in the form of a reactive ion etching (deep reactive ion etching, DRIE). As a result, second access openings 17 are formed in the silicon substrate 11. The etching of the second access openings 17 stops in the bulk silicon of the silicon substrate 11. This patterning step may also be used to create large area etch holes in other areas, for example, providing access to bondlands or saw trenches. In the further course of the process, turning off the passivation and sputtering components in the mentioned DRIE etching process with an undirected or isotropic

Further etching at the bottom of the second access openings 17 reaches an undercut. From an area with blind holes becomes in this way

freestanding grid without mechanical contact to the sensor core. One

Wire bonding or sawing trench area can be etched free in this step. FIG. 4 shows a consequence of said, changed etching regime during the formation of the second access openings 17. It can be seen that due to the changed etching parameters in the silicon substrate 11, spherical etching fronts have now grown together and thus form a second cavity 18. The second cavity 18 is separated from the remainder of the substrate by a grid formed in the silicon substrate 11 by the first access openings 17. The pressure sensor core is connected to the remaining substrate by means of springs 19, which have electrical leads (not shown). As a result, the all-round first access openings 14, the second cavity 18 and the continuous second access openings 17 for the sensor membrane 12 thus create an all-round release and thus a mechanical decoupling structure against externally acting mechanical stress. As a result, apart from electrical access and a mechanical fixation of the pressure sensor core, an all-round mechanical decoupling of the pressure sensor core is provided in order to advantageously minimize mechanical influences from the chip edge on the pressure sensor core.

Diameter of the second access openings 17 are preferably dimensioned such that suitable for media access of the pressure sensor

Access channels are created. The second access openings 17 in this way also realize effective protection of the pressure sensor core from particles and / or moisture (for example due to splash water). In a simple way was thus required for the said structure

Cavity in the form of the second cavity 18 provided by means of an etching process with modified etching parameters during the implementation. It can be seen that part-spherical surfaces are formed within the second cavity 18 as a consequence of the isotropic etching phase.

Fig. 5 shows a cross-sectional view through a micromechanical pressure sensor 100, which was prepared by an alternative method. It can be seen that in this case the second access openings 17 are formed with uniformly widening etching fronts, which extend in a trapezoidal manner downwards from the surface of the silicon substrate 11. This can be accomplished by controlling the above-mentioned DRIE process such that that individual etching fronts, which are defined by the second access openings 17 on the substrate rear side, converge in the course of the process and as a uniform etching front separate the pressure sensor core from the back or upper side of the silicon substrate 11.

As a result, as can be seen in FIG. 6, the second cavity 18 is thus formed against the surface of the silicon substrate 11 and laterally out of the trapezoidal etching fronts. As a result, etching gases can be more easily introduced in this way, and the entire etching process is advantageously well controllable.

FIGS. 7 and 8 show results of subsequent final processing steps for the micromechanical pressure sensor 100. FIG. 7 shows an electrical connection of the micromechanical pressure sensor 100 by means of an electrical feedthrough 41 and solder balls 50.

8 shows a micromechanical pressure sensor 100, which is encapsulated with molding compound, whereby a stable and resistant housing is provided for the micromechanical pressure sensor 100. Advantageously, a stress-decoupled pressure sensor with a cost-effective first wafer 10 with a silicon substrate 11 is thus provided. Use of costly substrates, e.g. in the form of an SOI substrate for the first MEMS wafer 10, is not required in this way. As a result, cost advantages can advantageously be achieved when producing a stress-decoupled micromechanical pressure sensor.

Preferably, the micromechanical pressure sensor 100 is formed as a piezoresistive pressure sensor, but is also conceivable realization as a capacitive micromechanical pressure sensor.

9 shows a basic sequence of a method for producing a micromechanical pressure sensor 100.

In a step 200, a MEMS wafer 10 is provided with a silicon substrate 11 and a first cavity 13 formed therein below a sensor silicon membrane 12. In a step 210, a second wafer 30 is provided.

In step 220, bonding of the MEMS wafer 10 to the second wafer 30 is performed.

Finally, in a step 240, an exemption of a sensor core 12, 13, 13 a from the back is carried out, wherein a second cavity 18 between the sensor core 12, 13, 13 a and the surface of the silicon substrate 1 1 is formed, the second cavity 18 by means of an etching process, which is carried out with defined changed Ätzparametern is formed.

In summary, the present invention proposes a method for producing a micromechanical pressure sensor with which a stress decoupling structure can be provided in a cost-effective manner. This is achieved by changing an etch regime when forming a second cavity within the first wafer above the pressure sensor membrane.

Although the invention has been described above by means of concrete examples of application, the person skilled in the art can realize previously or only partially disclosed embodiments of the invention without deviating from the gist of the invention.

Claims

claims 1 . A method of manufacturing a micromechanical pressure sensor (100), comprising the steps of:  Providing a MEMS wafer (10) having a silicon substrate (11) and a first cavity (13) formed therein beneath a sensor membrane (12);  Providing a second wafer (30);  Bonding a bottom surface of the MEMS wafer (10) to an upper surface of the second wafer (30); and  Cropping a sensor core (12, 13, 13 a) with bulk silicon (13 a), the sensor membrane (12) and the first cavity (13) from the top of the MEMS wafer (10) ago, wherein a second cavity (18) between the sensor core (12, 13, 13 a) and the surface of the silicon substrate (1 1) is formed, wherein the second cavity (18) by means of a  Etching process, which is carried out with compared to the formation of access holes (17) in the second cavity (18) used Ätzparametern defined modified Ätzparametern is formed. 2. The method of claim 1, wherein for forming the second cavity (18), the etching process is a reactive ion deep etching process with anisotropic onset and a defined isotropic continuation. 3. The method of claim 1 or 2, wherein for forming the second cavity (18) by means of a vertical etching process access openings (17) in the silicon substrate (1 1) are generated, wherein the vertical etching process is changed into a lateral etching process, wherein the lateral  Combining etching process resulting spherical etching fronts. 4. The method of claim 2 or 3, wherein after the reactive ion Tiefätzprozesses a passivation and sputtering of the etching process is turned off. 5. The method of claim 1, wherein the entire etching process is defined isotropically defined from the beginning. 6. The method of claim 5, wherein by means of the etching process downwardly widening, trapezoidal etching fronts are formed. 7. The method according to any one of the preceding claims, wherein the second wafer (30) is a passive substrate wafer or an ASIC wafer. 8. The method according to any one of the preceding claims, wherein the  micromechanical pressure sensor (100) is formed as a piezoresistive pressure sensor or as a capacitive pressure sensor. 9. A micromechanical pressure sensor (100), comprising:  a MEMS wafer (10) having a silicon substrate (1 1) and a first cavity (13) formed therein under a sensor membrane (12); a second wafer (30) bonded to a bottom of the MEMS wafer (10) with the MEMS wafer (10);  a sensor core (12, 13, 12) formed in a silicon substrate (11) 13a) with bulk silicon (13a) and a sensor membrane (12), wherein in the sensor membrane (12) a first cavity (13) is formed;  a second cavity (18) formed in the silicon substrate (11) above the sensor core (12, 13, 13a);  - wherein the second cavity (18) has been produced by means of an etching process whose etching parameters have been changed in a defined manner relative to etching parameters used to form access holes (17) in the second cavity (18). 10. Micromechanical pressure sensor (100) according to claim 9, characterized  in that, to form the second cavity (18), the micromechanical pressure sensor (100) has been formed by means of a reactive ion deep etching process with an isotropic onset and a defined isotropic continuation. 1 1. Micromechanical pressure sensor (100) according to claim 10, characterized  characterized in that the second cavity (18) of the micromechanical  Pressure sensor (100) is a piezoresistive or a capacitive pressure sensor.